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I just don't understand it...

s10er on Mon July 18, 2011 8:34 PM User is offline

Year: 1999
Make: Chev
Model: S10 Blazer
Engine Size: 4.3
Refrigerant Type: R134A
Ambient Temp: 90
Pressure Low: 29
Pressure High: 230
Country of Origin: United States

I'm a little mystified by the behavior of this critter. I got this old car last week to use on a rural mail route and it seemed that it was a little low on juice, so I sniffed it with a Yokogawa H-10 that's usually pretty sensitive to leaks. Only thing I could find was a small indication at the high pressure line from the compressor - not much and only at the end of an outer hose they installed over the h/p hose to protect it where it crosses over the fan shroud, so I concluded that some gas is making it's way through the hose and being collected in the outer protective hose and following to the end of it. There was also quite a bit of oil on the upper radiator hose and left valve cover immediately behind the compressor, but no gas leak indicated in that area by the sniffer. Looking closer I noticed that one end of the snap ring for the high limit switch in the rear of the comp. was out of the groove and the plastic cover of the switch was deformed as it was cocked it the hole behind the half attached snap ring, so I decided that that must be where the oil had come from. Pulled the comp. off it's mounts and got the snap ring reseated. Charged the system to the above pressures at 2000 r.p.m. and things seemed to work well. Still get a faint sniff of gas at the aforementioned hose, which I'll replace along with the high limit switch and whatever else as soon as I've decided what that else is.
Now here's the part I don't understand. The system cools great on the 20 mile trip to work in the morning as well as a couple hours later for about the first 20 minutes of stop and go on the route. Then the compressor stops for 2 or 3 minutes and comes back on for 10 - 15 minutes and goes off for a little longer before coming back on for a shorter time. After an hour or two it comes on occasionally for a short time with long intervals of blowing hot air. Get back to the office and spend 10 minutes checking in before heading home and as soon as I get on the highway it blows cold for the whole 20 mile trip home.
I had thought the high limit switch must be kicking it off in the heat of the stop and go on the route - or maybe the limit switch was damaged from being twisted in the half-attached snap ring, so I pulled the connector off and jumpered it but that seemed to make no difference.
I'm no refrigeration man, but I've begun to wonder if the genius who designed that headlight system that I can't turn off when I want to also designed an A/C system that shuts itself off in stop and go driving so the vehicle won't overheat.

GM Tech on Mon July 18, 2011 11:04 PM User is offline

You are experiencing high pressure cut-outs---from a poor air flow from a worn out clutch fan-- S-10s are famous for rotten clutch fans--put a new heavy duty one on it and be happy.

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

s10er on Mon July 18, 2011 11:37 PM User is offline

Thanks for the reply, GM!
That was a thought I had considered and dismissed after I jumpered the high pressure switch circuit as a check. Is there some other mechanism that cuts the comp. out in the absence of the high pressure switch?

mk378 on Tue July 19, 2011 12:04 AM User is offline

Intermittent operation could be the clutch gap. Once it cycles off normally on the low pressure switch it may not always restart, especially when hot. Test by hitting the clutch plate with a suitable tool while it is not cooling. If that makes the clutch pull in and start cooling again you need to reduce the gap.

s10er on Tue July 19, 2011 12:47 AM User is offline

Yes, I had one on another vehicle last summer that would start if you tapped it or if you hit a rough spot in the road while driving and decided there must be a loss of continuity in the clutch coil. New clutch fixed it.
I tried tapping (pretty vigorously) the clutch this afternoon while it was cut out, to no effect.
After I got home this afternoon I got it to cut out by making a few starts/stops and got the gauges hooked up just as it started pumping again. Showed something like 45 low side and a little over 300 high side. Let it sit and idle at about 2000 r.p.m. and the pressures gradually fell to about 32/230 and it sat there and ran like that for half an hour with no change - no external fan on the condenser - just maybe a 5 m.p.h breeze. It really seems to be high temp / high pressure, but how can it be high pressure cut out with the switch jumpered?

s10er on Tue July 19, 2011 12:59 AM User is offline

Another thought.
Is the clutch circuit controlled directly by the pressure switches or are the switches merely sensors for the ECM or some central logic device that reads the sensors and control switch positions and then makes the decision to activate or deactivate the clutch? I know that a lot of vehicles will deactivate the compressor under heavy acceleration demand. Could there be a malfunction in that mechanism causing this behavior?

mk378 on Tue July 19, 2011 8:14 AM User is offline

Finding the clutch seems to be OK the next thing to consider is the low pressure switch not re-closing every time. Try tapping on it, or unplug and jump it when not cooling and see if that makes the compressor start. You can't leave it jumped all the time because the evaporator would freeze up.

Hook up the high pressure switch again, that doesn't seem to be the problem and you might need it someday. The high pressure at idle especially slow idle is from the fan clutch like GM Tech said.

Edited: Tue July 19, 2011 at 8:16 AM by mk378

iceman2555 on Tue July 19, 2011 10:59 AM User is offlineView users profile

All in all very good suggestions. Considering the possible mileage of the vehicle.....GM is correct...get a new fan clutch on the darn thing. Be careful with the 'extreme duty' units, they are designed for heavy towing (yeah right.... in an S10 4.3....gimme a break), but they produce more air flow and are quite noisy. Stay with the factory replacement unit.
Check and adjust the air gap of the clutch as necessary. The clutch coil becomes heated and the coil resistance increases...thus the disengagement. Repair this issue
As for the leak....unless you are in the market for a new compressor....suggest to repair the leak ASAP. A undercharge will result in insufficient lubricant flow to the compressor and a compressor failure. There are so many issues that result from a loss of lubricant.....get the darn leak repaired.

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

s10er on Wed July 20, 2011 1:07 AM User is offline

Hi, guys,
Thanks for all the advise. Sorry to be so slow coming back, but I decided yesterday's advice was good enough to just take it all, and I just got in from the project.
I picked up the 'severe duty' fan clutch and a set of new hoses on the way home today, along with a new valve for the high pressure port (it wants to spew a bit when removing the hoses) and a high limit switch and snap ring. That high limit switch came out in crumbs and pieces. The air gap on the comp. clutch was a good .035, so I scooted it up to .015-.020 and got it all back together and added 28 oz. of magic and things were getting nice & cool when the compressor just turned off. No abnormal pressures, just stopped - and would not start again. Tapped on the limit switches, tapped on the comp. clutch. Nothing. Finally pulled the plug on the low pressure switch and jumpered it & things got busy again. That switch sits where it gets really warm. Warm enough I couldn't leave my hand on it for long, and half an inch away it's attached to the accumulator with ice water dripping off it. Well, it was an hour after closing time for the latest open parts store 20 miles away so I'll have to pick up a switch in the morning, but I'm pretty sure that was the central mystery with this deal. Won't know for sure until after tomorrow's mail run, but I'll fill you in then.
Decided to take a little drive to check things out (low pressure switch jumpered). Went down the road about 15 miles with the dial set on Max AC. Outside air temp. was between 86 and 89F all the way and the little dial thermomometer I had stuck in the vent just continued falling until I finally turned the cooler off at 29.0 F. I think it would have gone further, but I didn't bring a jacket.
By the way, that super-duper fan clutch really moves the air when it kicks in. I'm afraid to rev it up too much near the house - my wife has the tornado alarm set on the weather radio.

GM Tech on Wed July 20, 2011 7:33 AM User is offline

The problem is probably NOT the switch, but the connector to it---seen a bunch of them where they don't attach to the switch pins correctly- need more squeeze on the pins...

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

s10er on Wed July 20, 2011 7:49 AM User is offline

Will look closer.

s10er on Wed July 20, 2011 4:40 PM User is offline

Screwed a new low pressure switch on it this morning before going out on the route. Compressor wouldn't hit a lick with the old switch, but started right up with the new one and didn't skip a stitch all day. Now, if someone could just suggest a way to stuff mail in the boxes without lowering the window..... 104f on the way out of town this afternoon. Really nice to come home with dry clothes for a change.
Thanks again to all of you!

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